Polychloroprene is a commercially useful elastomeric polymer of 2-chlorobutadiene-1,3 (“chloroprene”) that has found utility for over seventy years in manufacture of rubber belts, hoses, engine mounts, adhesives and rubber profiles. The polymer may be a homopolymer, i.e. a polymer that is made up solely of copolymerized units of chloroprene monomer, or it may be a copolymer that contains copolymerized units of chloroprene and other comonomers. Because of the extremely high reactivity of chloroprene, it tends to homopolymerize. The amount of comonomer that can be introduced into the polymer chain by copolymerization is generally only about 2 mole percent in the case of tetrachlorobutadiene and even less for styrene and isoprene when overall monomer conversion is 65% and the initial level of comonomer present is 10 mole %. 2,3-dichlorobutadiene-1,3 is one of the few monomers that is more reactive than chloroprene itself. This characteristic has facilitated its use as a comonomer in the manufacture of a variety of polychloroprene copolymers, most of which have outstanding crystallization resistance and low temperature properties.
Synthesis of 2,3-dichlorobutadiene-1,3 may be accomplished by a number of methods that generally involve a dehydrochlorination reaction as a process step. For example, U.S. Pat. No. 4,215,078 describes a process for dehydrochlorination wherein the reaction is carried out with an aqueous mixture of sodium hydroxide and sodium chloride having the composition of a chlor/alkali cell liquor. U.S. Pat. No. 4,629,816 discloses a process for dehydrochlorination of 2,3,4-trichlorobutene-1 carried out under conditions where an alkali metal hydroxide solution is added to a mixture of the 2,3,4-trichlorobutene-1, phase transfer catalyst, inhibitor and water, while U.S. Pat. No. 6,380,446 describes the use of an upflow reactor in the dehydrohalogenation of various halogenated alkane and alkene compounds, including chlorinated butenes.
These dehydrohalogenation processes are typically conducted in the liquid phase by mixing the halogenated alkane or halogenated alkene with a strong base in a solvent. Because the base is usually added as an aqueous solution, phase transfer agents (also referred to as phase transfer catalysts) are usually employed to promote contact of the reactants. These catalytic materials promote reaction between reactants located in different phases by transferring one reactant across the interface into the other phase so that the reaction can proceed. The phase transfer agent is not consumed and performs the transport function repeatedly. See, e.g. Starks et al, “Phase-Transfer Catalysis”, Academic Press, New York, N.Y. 1978.
Although phase transfer catalysts are very effective at increasing conversion in some dehydrochlorination reactions, these higher conversions can result in increased formation of byproduct isomers that are difficult or impossible to remove from the desired product. In the dehydrochlorination of 3,4-dichlorobutene-1 to produce chloroprene for example, the reaction of some impurities in the organic reactant forms byproduct chlorobutadienes that contain chlorine substituents located at an alpha carbon (i.e. “alpha-chlorine”). As used herein, the term “alpha carbon atom” means a carbon at the end of a carbon chain (either an alkyl or alkenyl chain) generally numbered 1 in the IUPAC naming convention for alkanes and alkenes. By extension, a beta carbon atom is a carbon atom at the penultimate end of a carbon chain (“next to last” or second) generally numbered 2 in the IUPAC naming convention for alkanes and alkenes.
One method of controlling the problem of byproduct formation in dehydrochlorination reactions is by intentionally limiting reactant conversion to a level below that which is otherwise achievable. However, although intentionally limiting reactant conversion allows acceptable product purity to be attained, it also reduces yield and imposes an economic and environmental disadvantage that can be significant. In another example, high conversion conditions in the dehydrochlorination of 1,2,3,4-tetrachlorobutane to form 2,3-dichlorobutadiene-1,3 results in production of substantial amounts of isomeric dichlorobutadienes that contain alpha-chlorine substituents (i.e. chlorine atoms attached to alpha carbon atoms). The presence of isomeric products that contain alpha-chlorine substituents is objectionable because use of such mixtures as monomer feeds in polymerization reactions of 2,3-dichlorobutadiene-1,3, including copolymerization reactions with chloroprene, can result in formation of relatively high percentages of allylic chlorine in the polymer backbone. This can increase oxidative degradation of the polymer.
Removal of isomers containing alpha-chlorine substituents from desired products such as 2,3-dichlorobutadiene-1,3 that contain chlorine exclusively at beta carbons is difficult because the isomers generally have similar volatilities. Attempts made in the past to address this problem have not been completely satisfactory. For example, in U.S. Pat. No. 2,626,964 a method is described for increasing the purity of 2,3-dichlorobutadiene-1,3 formed via dehydrochlorination of 1,2,3,4-tetrachlorobutane by repressing the dehydrochlorination of 1,2,4-trichloro-2-butene formed in the reaction. Although the purity is increased somewhat, low overall yield and long reaction times detract from the commercial usefulness of this method.
Because of the difficulties associated with separation of isomers, conversion in dehydrochlorination reactions is typically intentionally limited to levels lower than those that are readily achievable in order to reduce formation of isomers containing alpha-chlorine substituents to acceptable levels.
It would be advantageous to have a method available that would permit attainment of high conversion in dehydrochlorination of 1,2,3,4-tetrachlorobutane as well as in the dehydrochlorination of compounds that can be used as intermediates in the formation of 2,3-dichlorobutadiene-1,3 while at the same time provide acceptable product purity.